This paper presents concepts of Trenton structure and of geologic history in Ohio, Indiana, and Illinois that are not commonly found in the literature. The literature is reviewed in order to show the origin of structural thought patterns, some of which have been preserved by repetition without any re-examination. This review shows great need for better understanding and for more careful use of structural terminology.

The Cincinnati Arch geologic province includes all the structure that separates the Appalachian, Michigan, and Illinois basins. The regional aspects of this structure have resulted from subsidence in the basins, rather than from uplift between the basins; consequently, there are no continuous structural axes. In the Appalachian basin subsidence began in pre-Trenton time. In the Michigan basin subsidence began in post-Niagaran Silurian time. In the Illinois basin subsidence began in Devonian time. There was no west dip in Indiana and, consequently, no Cincinnati arch before late Devonian time.

The major structural features in the Cincinnati Arch geologic province are: Cincinnati arch, Findlay arch, Francesville arch, and Indiana-Ohio platform. The three arches lose their identity in the platform which covers an area of 10,000 square miles in which there is no arched structure. Northern Illinois is in the Wisconsin Arch geologic province which had a different geologic history. The Oglesby fault is shown where the LaSalle anticline was first described. The upthrown block between this fault and the Sandwich fault is here shown as the Ottawa horst. Data for the interpretation of the Tuscola fault were supplemented by carefully located core-drill holes in three counties.

In Indiana the Logansport fault and the Francesville arch have been taken from a map contoured by use of 50-foot intervals. No arching extends from Indiana into Illinois, and none of the various "Kankakee arches" found in the literature will withstand the test of careful examination. The term "Kankakee arch" should be dropped.

Thickness of sedimentary columns between the top of the Trenton limestone and granite are used to show present structure does not reflect the configuration of the granite surface. The original configuration of the granite surface can be used to explain northward thinning from Kentucky to the Ontario Peninsula. Arkosic materials should not be used for age determinations or for long-range correlations. The first sediments above the granite in the Ontario Peninsula may have been deposited in Ordovician time.